Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

When handling a redirection event, we need to close the existing
connection before opening the new connection.

read_user() assumes that zero-length buffers don't exist, and optimises
around this.

separate xfer method.

Add missing .alloc_iob entries to several xfer_interface_operations
structures.

xfer_window() function, which can return a scalar rather than a
boolean.

implement sendto()/recvfrom() equivalents.

request(), seek() or deliver_xxx() in order to start the data flow.
Autonomous generators must be genuinely autonomous (having their own
process), or otherwise arrange to be called.  TCP does this by
starting the retry timer immediately.

Add some debugging statements.